Ironic, mood-lightening expression to be used upon bearing witness to an unpleasant shitstain or skidmark.

A prison guard examines an inmate's bed, peeling back the covers to reveal a large brown stain of fecal matter plastered over the sheets. To bring humour and gravitas to the discovery he remarks, "That's not gravy." (As seen on the television documentary 'Strangeways'.)